The present invention relates to a reflection type liquid crystal display device to be used in ornaments, clocks, electronic calculators, radios, small-sized mobile devices or displays. More particularly, the present invention relates to an especially bright reflection type liquid crystal display device which can convert an incident light partially into an electric power.
In the prior art, there is a display method which makes use of not a polarizing plate, but the light scattering properties of a liquid crystal, as represented by the phase transition mode or the polymer scattering mode. This light scattering mode causes no absorption loss of light by the polarizing plate but can make effective use of the light to make a bright display.
As the polymer scattering mode, for example, here is known a structure in which liquid crystal droplets are arranged in polymers, as disclosed in U.S. Pat. No. 4,435,047 or U.S. Pat. No. 4,688,900. However, these techniques adopt the droplet structure so that they fail to make the low-voltage drive and the sufficient scattering intensity compatible and the low-voltage drive and the sufficient transparency compatible. There is also known a structure in which the liquid crystals form a continuous layer having polymer materials distributed in a three-dimensional network structure, as disclosed in Japanese Patent Laid-Open No. 198725/1989.
FIG. 6 shows an example of the prior art, in which the black-and-white display is falsely made by a liquid crystal layer 3 using those light scattering mode.
As shown in FIG. 6A, the liquid crystal layer 3 is sandwiched between electrodes 2a and 2b on glass substrates 1a and 1b, and a reflection layer 4 is mounted on a substrate 7 below the liquid crystal layer 3. The reflection layer 4 is made of a metallic film of aluminum or silver having a high surface reflectivity. In this conventional example, when no voltage is applied to the liquid crystal layer 3 (that is, when OFF), the forward scatter in the liquid crystal layer 3 is reflected backward by the reflection layer 4 so that the white brightness increases to look white (or opaque) When the voltage is applied to the liquid crystal layer 3 (that is, when ON), the light scattering action in the liquid crystal layer 3 disappears so that the display is transparent to look falsely black. According to this method, the reflection layer has a mirror surface of high reflectivity so that the regularly reflected light dazzles the eyes of the observer, when the voltage is applied (or when ON). Thus, there arises a problem that the contrast between ON and OFF is degraded.
For solving the dazzling problem, on the other hand, it is possible to conceive a structure in which an absorption layer is provided in place of the reflection layer 4, as shown in FIG. 6B. Below the liquid crystal layer 3 sandwiched between the transparent electrodes 2a and 2b on the glass substrates 1a and 1b, there is mounted a light absorbing layer 5 on the substrate 7. The light absorbing layer 5 contains carbon or the like. According to this conventional example, when the voltage is applied to the liquid crystal layer 3 (or when ON), the light scattering action in the liquid crystal layer 3 disappears to establish a transparent display. As a result, the incident light is absorbed by the light absorbing layer to make a black appearance so that this sufficient black compensates the defect of the structure in FIG. 6A. When OFF, however, the backward scatter of the incident light in the liquid crystal layer 3 is too insufficient to keep the white brightness so that the white ground is dark. This also causes the problem that the contrast between ON and OFF is degraded.
For raising the white brightness, on the other hand, it is also possible to conceive a method by which the liquid crystal layer is thickened to increase the backward scatter in a polymer dispersed liquid crystal thereby to reduce the transmittance. However, this thickened polymer dispersed liquid crystal causes other problems that the responsibility is deteriorated and that the drive voltage is raised. This fails to satisfy the serious restricting condition of the low-voltage drive, as required for the use in clocks or mobile devices.
As means for coloring the light scattering mode, on the other hand, there is proposed a method by which a light absorbing plate for emitting a specific color is arranged on the light absorbing layer 5 in FIG. 6B. According to this method, however, the light absorbing plate at the back is seen through at the scattering time, too. This is not substantially different from the color of the light absorbing plate, as appearing at the transparent time, so that the visibility is poor. Thus, the method is defective in that it cannot make a full-color display by spatially mixing the basic color units of red, blue and green.
Here will be described an example of the construction of a small-sized mobile electronic device in which are packaged a reflection type LCD using the polarization plate of the prior art and a photovolatic element. FIG. 9 shows an appearance of a card-type electronic calculator. In FIG. 9, reference numeral 23 designates a display portion having the conventional reflection type LCD packaged therein; numeral 20 shows a solar cell portion; numeral 21 shows an input key portion; and numeral 22 shows a case. On the other hand, FIG. 10 shows an appearance of a digital wrist watch. In FIG. 10, numeral 23 designates a display portion having the conventional reflection type LCD packaged therein; numeral 20 shows a solar cell portion; and numeral 22 shows a case.
The following defects accompany the constructions of the small-sized mobile electronic devices in which are packaged the reflection type LCD using the conventional polarization plate and the photovolatic element. In the case of the card type electronic calculator of FIG. 9, for example, if the area of the display portion is enlarged to read the display easily, the area to be given to the solar cell portion is reduced to make the electric power insufficient for operating the card type electronic calculator. If the area of the key input portion is likewise enlarged to facilitate the inputting operation, the area to be given to the solar cell portion is reduced to make the electric power insufficient for operating the card type electronic calculator. Thus, it is preferable for the design for sufficing the intrinsic function of the card type electronic calculator that the area of the solar cell portion is as small as possible.
In the case of the digital wrist watch of FIG. 10, on the other hand, if the area of the display portion is enlarged to facilitate the reading of the display, the area to be given to the solar cell portion is reduced so that a sufficient electric power cannot be achieved for operating the digital wrist watch. Moreover, the presence of the large and blackish solar cell in the surface panel of the digital wrist watch makes the design rustic so that the fashion or an important factor of the watch is so poor as to degrade its commercial value.
In order to solve this problem, there is conceivable a method by which the reflection type LCD and the solar cell are overlapped. This method can be exemplified by mounting the solar cell on the back of the reflection type LCD. In the TN or STN mode having the construction of FIG. 8, however, the aluminum plate is used as the reflection plate but can hardly be expected for use in the transparent mode. When the reflection plate is made semitransparent, it is the current practice to vacuum-evaporate the aluminum thinly. Because of the absorption of aluminum, however, the transmittance is about 20% at most. As a result, the light, as could pass through the reflection type LCD and go into the solar cell at the back, is reduced to as low as 5% of the value for the absence of the reflection type LCD. On the other hand, the brightness of the reflection type LCD is about one half of that of the usual case where the aluminum reflection plate is used. As a result, the brightness of the reflection type LCD is so dark as to provide a poorly visible display. Moreover, the electric power, as generated by the solar cell, is as low as 5% of the value for the absence of the reflection type LCD so that it cannot operate the small-sized mobile electronic device normally.
In the pseudo black-and-white display reflection type liquid crystal display device using the light scattering type liquid crystal of the prior art, as described hereinbefore, when a reflection plate of high reflectivity is used, it dazzles the observer's eyes with the regular reflection, when the voltage is applied (ON). As a result, there arises a problem that the black display is hard to make so that the visibility is too poor to give aclearcontrast. When the reflectionplateis replacedbythelight absorbing layer so as to make the black display, on the other hand, the white brightness by the backward scatter cannot be achieved, when no voltage is applied (OFF). This raises another problem that the display is too dark white to give a clear contrast. In addition, there exists no means for making the sufficient contrast and brightness compatible for the color display.
Moreover, it is difficult to provide a reflection type LCD of high power generating efficiency, which is freed from reductions in the brightness and visibility of the reflection type LCD using the conventional polarization plate even when the reflection type LCD and the solar cell are overlapped.